Radiation sensitive silver halide emulsions containing one or a combination of chloride, bromide and iodide ions have been long recognized to be useful in photography. Each halide ion selection is known to impart particular photographic advantages. Although known and used for many years for selected photographic applications, the more rapid developability and the ecological advantages of high chloride emulsions have provided an impetus for employing these emulsions over a broader range of photographic applications. As employed herein the term "high chloride emulsion" refers to a silver halide emulsion containing at least 50 mole percent chloride and less than 5 mole percent iodide, based on total silver.
During the 1980's a marked advance took place in silver halide photography based on the discovery that a wide range of photographic advantages, such as improved speed-granularity relationships, increased covering power (both on an absolute basis and as a function of binder hardening), more rapid developability, increased thermal stability, increased separation of native and spectral sensitization imparted imaging speeds, and improved image sharpness in both mono- and multi-emulsion layer formats, can be realized by increasing the proportions of selected tabular grain populations in photographic emulsions.
These various photographic advantages were associated with achieving emulsions in which tabular grains having {111} major faces account for at least 50 percent of total grain projected area, hereinafter referred to as {111} tabular grain emulsions. Although the art has succeeded in preparing high chloride {111} tabular grain emulsions, the inclusion of high levels of chloride as opposed to bromide, alone or in combination with iodide, has been difficult. The basic reason is that tabular grains are produced by incorporating parallel twin planes in grains grown under conditions favoring {111} crystal faces. The most prominent feature of tabular grains are their parallel {111} major crystal faces.
To produce successfully a high chloride tabular grain emulsion two obstacles must be overcome. First, conditions must be found that incorporate parallel twin planes into the grains. Second, the strong propensity of silver chloride to produce {100} crystal faces must be overcome by finding conditions that favor the formation of {111} crystal faces.
Through extensive investigations organic compounds, hereinafter referred to as grain growth modifiers, have been identified that efficiently produce high chloride {111} tabular grain emulsions. Aromatic N-heterocyclic compounds capable of acting as grain growth modifiers include those disclosed by Maskasky U.S. Pat. No. 4,400,463 (e.g., adenine), Maskasky U.S. Pat. No. 4,713,323 (e.g., 4-amino-pyrazolo[3,4-d]pyrimidine), Tufano et al U.S. Pat. No. 4,804,621 (e.g., 2,4-diamino-1,3,5-triazine), Maskasky U.S. Pat. No. 5,178,997 (e.g., 7-azaindole), Maskasky et al U.S. Pat. No. 5,178,998 (e.g., xanthine) and Maskasky U.S. Pat. No. 5,185,239 (e.g., 4,5,6-triaminopyrimidine). Dicationic bipyridinium salts capable of acting as grain growth modifiers include those described by Marimoto U.S. Pat. No. 4,983,508 (e.g., 1,1'-dibenzyl-4,4'-bipyridinium dichloride). Sulfur containing organic compounds capable of acting as grain growth modifiers include those described by Takada et al U.S. Pat. No. 4,783,398 (e.g., 2,4-thiazolidinedione) and Nishikawa et al U.S. Pat. No. 4,952,491 (e.g., 5-(3-ethyl-2(3)benzothiazolylidene)-3-.beta.-sulfoethyl-rhodanine).
Maskasky U.S. Pat. No. 5,298,387 teaches to prepare a high chloride {111} tabular grain emulsion in the presence of an N-heterocyclic grain growth modifier, specifically a 2-hydroaminoazine. Thereafter the adsorbed 2-hydroaminoazine can be released from the tabular grain surfaces and replaced by a 5-iodobenzoxazolium compound. Although 5-iodobenzoxazolium compounds are capable of stabilizing high chloride {111} tabular grains once formed, these compounds cannot be relied upon as grain growth modifiers to produce high chloride {111} tabular grains in the course of emulsion precipitation.
Pollet et al U.S. Pat. No. 3,982,947 discloses as useful antifoggants iodobenzenes substituted with a hydroxy or carboxy group and from 1 to 4 substituents chosen from the group consisting of hydrogen, halogen, alkyl, alkoxy, alkoxycarbonyl, sulfo, aryl, fused-on benzene, hydroxy and carboxy.